Quick answer

If your water heater won’t drain, it is very likely to be plugged up with sediment.

If this is a water heater that is currently working and you are simply trying to do a flush, LEAVE IT ALONE!

If you open the drain when it hasn’t been used in a long time, you might not be able to get it closed again! Worse yet, if it is an old water heater, dislodging the sediment might actually cause a leak, and you’ll wind up having to replace the entire water heater.

On older units that haven’t been regularly flushed, it is best just to leave them be and plan for a replacement in the future when the heater eventually stops working. Rheem gives the same warning: tanks that have not been flushed in a while may have hardened sediment, and flushing them yourself can lead to leaks or further damage.

However, if you are trying to replace a water heater that won’t drain, there are a couple of things you can do.

Before you try any of these, turn off the power or gas and the cold water supply, and be careful: the water in the tank can be hot enough to burn. To start, make sure the hot water faucets are open, and the pressure relief valve has been flipped up.

Try to push a stiff wire up through the drain to unclog the sediment (stand to the side: hot water and sediment can come out suddenly)
Back flush with a washing machine hose (double-ended) connected to a threaded faucet
Use a pump

If the water heater is draining slowly, it might be easier to wait it out. It will drain as much as it can, but you will still likely have sediment in the bottom of the tank, making it heavy and difficult to transport. If you can get it outside, you can knock the valve off, and the rest of the contents will drain quickly. This is not advisable if the water heater is still in the house, however. You will absolutely flood the utility room.